The fossilized giant teeth of ground sloths reveal their unique role in the prehistoric ecosystem

Imagine a sloth. You probably imagine a medium-sized, arboreal creature hanging from a branch. Commonly depicted on children’s backpacks, stationery and lunch boxes, today’s sloths are slow-moving creatures that live inconspicuously in the rainforests of Central and South America.

But their giant Pleistocene ancestors, who lived in the Americas as early as 35 million years ago, were nothing like the dormant tree-huggers we know today. The giant ground sloths—some weighing thousands of pounds and standing taller than a one-story building—played very important and diverse roles in shaping ecosystems across the Americas, and those roles disappeared at the end of the Pleistocene.

In the new study, published in the journal Biology Letters, we sought to reconstruct the diets of two species of giant sloths that lived side by side in what is now Southern California. We analyzed remains recovered from the La Brea tar pits, colloquially known as the Shasta ground sloth (Nothrotheriops shastensis) and Harlan’s ground sloth (Paramylodon harlani). Our work sheds light on the lives of these fascinating creatures and the ecosystem consequences of their extinction in Southern California 13,700 years ago.

Challenges of Dentin Teeth

When studying the diets of extinct animals, it often seems like the jigsaw puzzle is only part of the puzzle. Stable isotope analysis has revolutionized the way paleoecologists reconstruct the diets of many ancient organisms. By measuring the relative ratio of light and heavy carbon isotopes in tooth enamel, we can find out what kind of food the animal ate – for example, grass and trees or shrubs.

But giant sloths’ teeth lack enamel, the highly inorganic and hard outer layer on many animals’ teeth, including our own. Instead, sloth teeth are made mostly of dentin, a more porous and organic-rich tissue that easily changes its chemical composition when fossilized.

Stable isotope analysis is less reliable in sloths because the chemical composition of dentin can be altered postmortem, distorting isotopic signatures.

Another method scientists use to glean information about an animal’s diet relies on microscopic wear patterns on its teeth. Analysis of the microwear texture of the teeth can determine whether the animal ate mostly hard foods such as leaves and grass or hard foods such as seeds and fruit pits. This method is also tricky when it comes to petrified sloth teeth, as signs of wear may persist differently on softer dentin than on harder enamel.

Before studying fossil sloths, we checked tooth microwear techniques in modern xenarthrans, a group of animals that includes sloths, armadillos, and anteaters. This study showed that microscopic wear of dentin can reveal dietary differences between leaf-eating sloths and insect-eating armadillos, giving us confidence that these tools can reveal dietary information from terrestrial sloth fossils.

Revealed exclusive nutritional niches

Previous studies have suggested that giant ground sloths were either grass-eating grazers or leaf-eating browsers based on the size and shape of their teeth. However, direct nutritional measures such as stable isotopes or dental microablation were often lacking.

Our new analyzes revealed contrasting signs of tooth wear between two co-occurring ground sloth species. The Harlan ground sloth, the larger of the two, had microwear patterns dominated by deep pit textures. This wear and tear indicates that you are chewing hard, mechanically challenging foods such as tubers, seeds, mushrooms, and fruit stones. Our new evidence is consistent with skeletal adaptations that indicate powerful digging abilities consistent with both above- and below-ground foraging.

In contrast, the Shasta ground sloth exhibited a tooth microwear texture more similar to that of leaf-eating and woody herbivores. This pattern supports previous studies of fossilized dung showing a diet rich in desert plants such as yucca, agave, and salt.

Next, we compared the texture of sloth microclothes to those of ungulates such as camels, horses, and bison that lived in the same region of Southern California. We confirmed that the feeding behavior of none of the sloth species completely overlapped with that of other herbivores. Giant ground sloths did not perform the same ecological functions as other herbivores that shared their landscape. Instead, the two ground sloths shared their niches and played complementary ecological roles.

Extinction brought ecological losses

Harlan’s ground sloth was the engineer of a megafaunal ecosystem. It dug up the soil and foraged underground, affecting soil structure and nutrient cycling, even dispersing seeds and fungal spores over large areas. Anecdotal evidence suggests that some anachronistic fruits, such as the strangely bumpy texture and softball-sized Oaged oranges, were dispersed by ancient megafauna, such as giant ground sloths. When the Pleistocene megafauna disappeared, the loss contributed to the regional limitation of these plants, as there was no one nearby to spread their seeds.

The broader implication is clear: megafaunal extinctions wiped out critical ecosystem engineers, causing cascading ecological changes that continue to affect habitat resilience today. Our results resonate with growing evidence that conserving today’s large herbivores and understanding the diversity of their ecological niches is critical to preserving functional ecosystems.

Studying the teeth of the extinct giant sloths revealed not only their diet, but also the long-term ecological legacy of their extinction. Today’s sloths, though fascinating, only hint at the great environmental impact of their prehistoric relatives, giants who shaped the landscape in ways we are only beginning to appreciate.

This article is republished from The Conversation, a not-for-profit independent news organization that provides facts and sound analysis to help make sense of our complex world. Written by: Larisa RG DeSantis, Vanderbilt University and Aditya Reddy Kurre, University of Pennsylvania

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Larisa RG DeSantis has received funding from the National Science Foundation, the Guggenheim Foundation, and Vanderbilt University. DeSantis is also a research associate at the La Brea Tar Pits and Museum.

Aditya Reddy Kurre received funding from Vanderbilt University.